1. Field of the Invention
This invention relates to dust cores for use as magnetic cores in transformers and inductors, cores in motors, and other electromagnetic parts, ferromagnetic powder compositions for forming the dust cores, and a method for preparing the dust cores.
2. Prior Art
In the prior art, silicon steel lamination cores having punched silicon steel sheets stacked are often used in inductance elements of electronic devices. The lamination cores, however, are difficult to automate a manufacturing process. Especially when cores for motors and other drive equipment are prepared by punching from sheets, the material yield is extremely low because such cores have a complex shape. To fabricate three-dimensional shapes, a great number of working steps is necessary.
There are known dust cores or powdered-iron cores wherein ferromagnetic metal powder is bound with a binder such as water glass. Iron powder, permalloy powder and sendust powder are typical of the ferromagnetic metal powder. Dust cores can be integrally formed and worked even if they are of complex shape. The material yield is substantially 100%. The dust cores are expected to become a substitute for the lamination cores.
The ferromagnetic alloy powders such as permalloy powder and sendust powder, however, cannot be a substitute for the silicon steel lamination core commonly used in drive equipment because these powders have a low magnetic flux density despite a low coercivity.
With respect to iron powder, there are commercially available different forms of iron powder prepared by various processes such as electrolytic decomposition and water atomization processes. They have a coercivity of more than 2 Oe which is not so low as comparable to silicon steel. Gas atomized iron powder has a coercivity of about 1 Oe, but is extremely expensive and thus inadequate as a substitute for the silicon steel lamination core.
A number of proposals have been made for improving the characteristics of dust cores.
For example, Japanese Patent Application Kokai (JP-A) 72102/1987 discloses an iron powder for dust cores having an oxygen content of 0.15 to 0.5% by weight, a mean particle size of 40 to 170 .mu.m and an average aspect ratio of 4/1 to 25/1. Oxide coatings on iron particles provide for insulation between particles to reduce eddy current losses. The oxygen content is relatively high because the target is a high frequency band of higher than about 1 MHz. Since dust cores are prepared using an epoxy resin binder, annealing treatment at high temperature for reducing coercivity is precluded, resulting in dust cores having increased hysteresis losses.
JP-A 824027/1986 discloses in Examples iron cores which are prepared by mixing an iron powder having a mean particle size of 54 .mu.m with a titania powder having a mean particle size of 0.3 .mu.m or a zirconia powder having a mean particle size of 1 .mu.m and pressure molding the mixture. JP-A 260005/1988 discloses a magnetic core which is prepared by adding silicon oxide having a particle diameter of up to 1 .mu.m to an iron powder of -200 mesh. These dust cores, however, have several problems including (1) substantial core losses, (2) low magnetic flux densities because large amounts of insulating material are needed for insulation, (3) difficult lowering of coercivity because they cannot be annealed at high temperature and the strain created during molding is not fully relaxed.
To comply with the recent trend toward the size reduction of electric and electronic equipment, dust cores are required to be compact and efficient. Cores of ferromagnetic metal powder can be reduced in size owing to the high saturated magnetic flux density of the powder, but substantial eddy current losses occur because of the low electric resistance. Then ferromagnetic metal particles are often covered on the surface with insulating coatings. In the dust core manufacturing process, annealing is usually effected in order to release the strain or stress created during molding and to reduce the coercivity of dust cores. Annealing must be done at high temperature in order to fully relieve ferromagnetic metal particles from stresses. However, since water glass or a similar insulating material experiences a substantial loss at high temperature, high temperature annealing results in insufficient insulation among ferromagnetic metal particles. This, in turn, results in substantial eddy current losses in the high frequency region, exacerbates the frequency response of magnetic permeability, and increases the core loss. No satisfactory magnetic properties are obtained.